Since ch has already been de-referenced by the time we get to the BUG_ON,
it is useless. The back trace alone is enough to tell what is going on,
delete the redundant BUG_ON.

Link: https://lore.kernel.org/r/20191217194437.25568-1-pakki001@umn.eduSigned-off-by: Aditya Pakki <pakki001@umn.edu>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>